The invention relates to vibratory seismic energy sources used for siesmic prospecting, and more particularly, relates to a means for measuring the actual force that is imparted to the earth surface by a seismic vibrator.
In seismic prospecting vibratory sources, also called seismic vibrators, are used to impart vibratory seismic energy into the earth. An example of such a seismic vibrator has been described in U.S. Pat. No. 4,184,144.
Since seismic vibrators are known in the prior art, they will not be described in detail. Generally, it can be said that a vibrator has a piston affixed to a coaxial piston rod. The piston is located in a cylinder formed within a heavy reaction mass. Means are included for alternately introducing hydraulic fluid under high pressure to opposite ends of the cylinder, thereby imparting a motion to the piston relative to the reaction mass. These means are actuated by a pilot signal and may, for example, comprise a power valve, a pilot valve, a pump, a torque motor, etc. The piston rod extending from the reaction mass is rigidly coupled to a ground coupling (also called base plate) which is maintained in contact with the ground. The inertia of the reaction mass tends to resist displacement of the reaction mass relative to the earth. The piston motion is coupled through the piston rod and base plate to impart vibratory seismic energy into the earth.
Several ways of mounting a vibrator exist. One example of a suitable mounting is vehicle mounting. When a vibrator containing vehicle has arrived at a selected location, the base plate is lowered into contact with the ground and to prevent decoupling of the base plate from the ground during operation, a portion of the vehicle's weight is applied to the base plate. The vehicle's weight is applied to the base plate through spring members in such a way that a static bias force is imposed on the base plate, while the dynamic forces of the base plate are decoupled from the vehicle.
The vibrator can be driven in any suitable way as is well known to those skilled in the art and thus will not be described in detail.
Generally, it can be said that the force exerted on the earth surface by the base plate of the vibrator, the co-called ground-force, determines seismic phenomena occurring at a large distance from the vibrator. In other words, the ground-force can be considered as an output signal of the seismic vibrator.
In order to be able to derive desirable information about the ground-force such as amplitude- and phase-spectra, it is necessary to measure accurately the actual ground-force of the seismic vibrator. Conventional methods for determining the ground-force of a seismic vibrator have been based upon determining the masses and accelerations of the reaction mass and base plate construction respectively, and deriving from these quantities the ground-force of the seismic vibrator by assuming a balance of forces: EQU F ground=-M.sub.m .multidot.a-M.sub.bp .multidot.a.sub.bp
in which
M.sub.m =mass of reaction mass PA1 a.sub.m =acceleration of reaction mass (positive sign, if directed towards the earth surface) PA1 M.sub.bp =mass of base plate PA1 a.sub.bp =acceleration of base plate (positive sign, if directed towards the earth surface)
However, from experiments it appeared that the ground-force derived in the above manner could not be considered as a correct representation of the acutual ground-force imparted to the earth surface since the above methods for determining the ground-force from the balance of forces assume that the base plate is a rigid base plate. However, it appears that such an assumption is not always correct.
Base plate bending causes parts of the base plate to move with different accelerations. To compensate for this phenomenon an effective base plate acceleration should be defined that differs from the acceleration at a particular point, or an effective base plate mass should be used that is different from the true base plate mass. Another cause for the above discrepancy is the force loss through isolation bags used for decoupling the vehicle from the baseplate.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, an improved apparatus is provided for measuring the force which is actually exerted by the base plate of a seismic vibrator on the earth surface.